Subwooferin'...newbee seeks advice.

I recently acquired a pair of Apogee Slant 6's (6.5" Vifa woofer crosses over to a 26" ribbon at 1150 Hz) that I placed in my 22.5' x 24.5' room. I'm quite happy with the overall sound except, as you would guess, for bass reach/limitations. While the little woofers sound pretty good within their limits there are some CDs in my collection which overstress them. I'm investigating which subwoofers might work best with them and am thinking something like a powered pair with high pass filters built in, perhaps using 10" or 12" drivers, would be quick enough and would keep the very lowest frequencies out of the Apogees.

I'm using a Plinius 8150 integrated amp which has identical pairs of speaker outputs and also the "preamp out" feature.

Can anyone recommend a subwoofer with transparent high pass filters that won't affect, or will have very little affect, on the qualities of the main speakers? Thank you.
I don't understand how high-pass filters can be utilized with your Plinius 8150 unless it has an "amp in" input -- you only mentioned the "preamp out" jack.

Also, I would suggest getting a separate active HPF that can be placed close to the 8150. Typically, the sub will be some distance from the 8150 so you'd be running two long interconnects to and from the sub.

M&K makes bass management controllers that are transparent enough for studio work -- BMC Mini (unbalanced) and LFE-4 (balanced). I use both in separate systems. You'll also find excellent subs from M&K.

Hope this helps.
Thanks for your comments, Bob. I'm thinking I could hook up the subwoofer to one set of speaker cables from the amp, and then after running through the high pass filter in the subwoofer that signal can go from the subwoofer to the Apogees' woofer inputs. I could then run my second set of speaker cables from the amp directly to the Apogees' midrange/tweeter inputs (the speakers are bi-wireable). Does this sound do-able?

Now to find a subwoofer that will do this with minimal effect on the sound of the main speakers.
Sorry, I didn't understand you were needing speaker level connections. Take a look at the MX-7000 here:

I think it (and other models) will do what you wish.